ToolTips in a CWND?

I'm currently sending a SendMessage command
to the tooltip object I get from _AFX_THREAD_STATE
with a ti structure that points to a child window.
The SendMessage returns TRUE for TTM_ADDTOOL, but
I never get a TTN_NEEDTEXT message for anything
but the main window.
I am setting the lpszText field to LPSTR_TEXTCALLBACK;

(The main example I'm going off of is on page 623
of Charles Petzold's "Programming Windows 95."